Evapotranspiration

Evapotranspiration is the process of evaporating water from leaves through plant transpiration during photosynthesis. It varies because of a multitude of factors like wind, temperature, humidity, and water availability.

Evapotranspiration is an important process in the water cycle because it is responsible for 15% of the atmosphereâ€™s water vapor. Without that input of water vapor, clouds couldnâ€™t form and precipitation would never fall. Evapotranspiration is the combined name for the processes of evaporation and transpiration. When water vapor is released into the atmosphere both processes are involved, so they have been combined into one word to cover all bases.

The evaporation in evapotranspiration refers to water evaporated from over land. This includes evaporation from soil, wetlands, and standing water from places like roofs and puddles. It can also refer to direct evaporation of liquid water from the leaf surface of the plant.

Transpiration happens when plants release water vapor from tiny holes, called stomata, in their leaves. This is caused in part by the chemical and biological changes that occur as the plant undergoes photosynthesis and converts carbon dioxide into oxygen. Transpiration performs the same function as human sweating because plants do it to cool down their leaves. Figure A shows the stomata on the underside of the leaf releasing water vapor (blue arrow) because of the warmth from the sun.

There are a whole host of factors that affect evapotranspiration:

Temperature â€“ As temperature increases, the rate of evapotranspiration increases. Evaporation increases because there is a higher amount of energy available to convert the liquid water to water vapor. Transpiration increases because at warmer temperatures plants open up their stomata and release more water vapor.

Humidityâ€“ If the air around the plant is too humid, the transpiration and evaporation rates drop. Itâ€™s the same reason sweat does not evaporate from our skin when itâ€™s too humid.

Wind speed â€“ If the air is moving, the rate of evaporation will increase. The wind will also clear the air of any humidity produced by the plantâ€™s transpiration, so the plant will increase its rate of transpiration.

Water availability â€“ If the soil is dry and there is no standing water there will be no evaporation. If plants canâ€™t get enough water they will conserve it instead of transpiring by closing their stoma.

Soil type â€“ Soil type determines how much water soil can hold and how easy it is for the water to be drawn out of it, either by a plant or by evaporation. For areas where the ground is covered by vegetation, the rate of transpiration is considerably higher than the rate of evaporation from the soil.

Plant type â€“ Some plants, like cacti and other succulents, naturally hold onto their water and donâ€™t transpire as much. Trees and crops are on the other end of the spectrum and can release copious amounts of water vapor in a day. For example, an acre of corn can release 4,000 gallons of water vapor a day and a single large oak tree can transpire 40,000 gallons of water vapor in a year.

If you can predict evapotranspiration rates, you will be able to estimate the water demands of the crop. This may help you to determine whether or not to irrigate, for example. If crops do not receive enough water, their leaves may curl and their production decline as the plants fight to conserve what water they can. Knowledge of predicted temperature and wind conditions from weather forecasts can give you a clue to how strong the evapotranspiration rates will be.

Evaporation may also directly affect soil moisture conditions. If there is too much moisture in the soil, the farm machinery can get bogged down because it has to work too hard. The weight of the machinery can also compact the wet soil, leading to lack of air for healthy root systems to develop. If the soil is too dry, however, the plants may be easily stressed due to the lack of available water and a crust may sometimes form on top of the soil. This crust may be so impermeable that when it rains on top of the crusty soil, the rain runs right off rather than soaking in.